Group B streptococci (GBS) are a significant cause of invasive bacterial disease in human neonates, parturient women, and patients with underlying illnesses. Bacterial virulence factors that contribute to the pathogenesis of GBS disease have not been completely defined. The purpose of the proposed research is to identify and characterize virulence factors contributing to disease caused by two of the most common GBS serotypes, type I and III, that produce disease in humans. One putative GBS virulence factor is C5a-ase, an enzyme that rapidly inactivates the potent polymorphonuclear (PMN) chemoattractant C5a. This enzyme could contribute to the virulence of GBS by reducing PMN accumulation at the site of bacterial invasion, thereby permitting bacterial multiplication and systemic invasion. The first aim of this proposal is to determine if antibodies to C5a-ase can neutralize the C5a-ase activity on type I GBS or initiated opsonization of GBS in the presence of complement, and to determine the contribution of C5a-ase to the virulence of type I GBS. Work presented in this proposal indicates that the majority of invasive type III GBS infections are caused by a group of closely related organisms, called RDP III-3, based on the similarity of the restriction digest patterns (RDPs) of bacterial DNA following enzymatic digestion with Hind III and Sse 83871. Unique DNA sequences that are present only in III-3 strains have been identified by subtractive hybridization of genomic DNA from III-3 and III-2 strains. In the work proposed in Aim 2, the III-3 genes in which these sequences are contained will be characterized (cloned and sequenced). The resulting data will delineate the genetic differences between the virulent and avirulent RDP types, and disclose the mechanisms by which GBS become genetically divergent and become virulent. Genes that are unique to III-3 strains are likely to encode virulence factors that increase the pathogenicity of the III-3 strains. In the third and final aim, candidate virulence genes will be inactivated in the virulent III-3 strains, and avirulent III-2 strains will be transformed with these virulence genes, in order to determine their role in disease pathogenesis. This approach will ultimately contribute to understanding the molecular basis of disease caused by GBS, and help devise strategies for preventing and treating this important cause of serious human neonatal bacterial infection.